The present invention relates to low-density wood strand board panel and a method of manufacturing such panels.
Strand boards more particularly oriented strand board (OSB) are and have been used in construction for more than 20 years. In such construction, such boards or panels are particularly suitable for sheathing and are commonly provided in 4 ftxc3x978 ft. These sheets provide significant competition to plywood panels, which offer a high degree of strength. At low density, the density of plywood panels is in the range of 28 to 32 lbs/ft3 or about 450 to 510 kg/m3.
Oriented strand, boards were initially developed as an alternative to plywood panels. These alternative boards are composed of wood fragments (strands) that are produced from low grade round wood. As is well known an oriented strand board (OSB) panel is formed by layering flakes or strands of wood mixed and coated with a resin or binder (thexe2x80x9cfurnishxe2x80x9d) and wax, to form a three layers mat (generally referred to as a lay-up) and pressing the mat under heat and pressure to form a consolidated board. The strands moisture content is controlled to provide the desired moisture content so as to create conditions in the press that avoid such phenomenon as blow back (explosions in the board as the consolidating pressure is released). To produce a consolidated panel having similar strength characteristics with plywood (generally slightly lower) requires the use of more material and produces a higher density product but at a lower materials cost.
Examples of OSB products are provided in the following US Patents: U.S. Pat. No. 4,246,310 (Hunt et al.); U.S. Pat. No. 4,610,913 (Barnes); U.S. Pat. No. 5,506,026 (Iwata et al.); U.S. Pat. No. 5,736,218 (Iwata et al.); and U.S. Pat. No. 6,129,871 (Suzuki et al.).
The typical density of an OSB panel is in the range of 600 to 640 kg/m3, which is considerably higher than that for plywood. In the result, the weight of OSB panels is also considerably greater. It has long been the desire of those in the OSB industry to provide a low density OSB having the required strength requirements. Such an accomplishment will not only reduce the unit cost for a given thickness, but will also make using (handling) of the board significantly easier. For this reason, various attempts have been made to provide a xe2x80x9clow densityxe2x80x9d OSB, that is, a panel that is similar in density to plywood, while still preserving the desired strength characteristics.
A significant component to total production time in the manufacture of OSB and the limiting factor in most plants is the total press time required to consolidate the mat or lay-up to form the consolidated OSB panel. The total press time required to produce a properly consolidated panel is increased with thickness of the panel and, generally about 140 to 180 seconds is required for the 11.1 mm ({fraction (7/16)}) panels. Reducing this time requirement has been a goal of the industry for some time. The prior art, such as U.S. Pat. No. 6,129,871, (""871) provides two methods of providing moisture to an OSB furnish. The first involves the injection of steam into the furnish during the pressing phase. This process requires complex steam injection equipment and specially designed platens in the press to provide the steam into the furnish. As will be understood, this type of process leads to increased production costs. An alternative to the steam injection process is the addition of water to the furnish to increase the moisture content. For example, in the ""871 patent, the moisture content of the wooden strands in the furnish is first adjusted to about 10 to 25 wt % prior to pressing. The final panel results in a moisture content of 5 to 15 wt %.
As mentioned above, one of the industry goals is to provide a low density OSB panel that can be made with less wood while maintaining the desired strength characteristics of the final product. Simply reducing the wood content of the furnish (i.e. reducing the density of the furnish) was found to lead to an increase in air voids in the furnish and resulting in lower heat conductivity during pressing. One solution to this problem was to provide more moisture to the furnish. However, this was found to result in further problems. Firstly, a furnish having a higher moisture content was found to require higher press platen temperatures and longer cycle times (i.e. time within the press) since a greater volume of water needed to be converted to steam. Secondly, the increased moisture content and associated increased steam generation resulted in extremely high pressures within the panel. In the result, once the press was opened, the rapid release of pressure caused a xe2x80x9cblow-outxe2x80x9d, or explosive expansion of the boards. Such blowouts result in loss of production time, waste of material, and potentially dangerous conditions for workers.
Fahrni in U.S. Pat. No. 2,686,143 attempted to make a strandboard using different types of strands in the core than in the surface layers and different amounts of binder in the core and surface layers i.e. significantly higher binder content in the surface layers than in the core. It is not clear how successful this attempt may have been to produce a low-density board, but the amount of resin required in the surface layers makes the cost of such a board non-competitive.
Elemdorf a pioneer in the OSB field attempted to improve OSB strength characteristics by using cement as the binder (see U.S. Pat. No. 3,164,511).
Chapman (one of the originators of the waferboard industry) in U.S. Pat. No. 2,061,878 teaches the use of green wood strands (not dried) to produce a superior product as compared with one produced from dried wood strands.
Therefore, there is a need for a process for producing low-density OSB panels that avoids at least some of the problems in the known methods. The present invention seeks to address this need.
It is an object of this invention to provide a method of producing a low-density OSB panel having acceptable strength characteristics and to the product so produced:
Broadly the present invention relates to a method of producing a strand board having an average density of between 450 and 550 kg/m3 comprising: conditioning a first set of strands to a moisture content of between 20 and 50% based on the dry weight of said strands and coating a bonding resin to the surfaces of said strands to provide a first set of resinated strands, conditioning a second set of strands to a moisture content of between 2 to 15%, based on the dry weight of strands in said second set of strands and coating a bonding resin to the surfaces of said strands of said second set of strands to provide a second set of resinated strands, forming a mat lay-up having a core layer formed of said second set of resinated strands sandwiched between a pair of surfaces layers each formed from said first set of resinated strands, pressing said mat lay-up between heated platens to form a consolidated panel having an average density of between 450 and 550 kg/m3, said pressing including closing said platens to the thickness of said consolidated panel in less than 30 seconds and raising temperature of said core layer to 100xc2x0 C. in less than 35 seconds without increasing the core gas pressure above 0.17 MPa.
Preferably, said closing said platens to panel thickness in less than 20 seconds.
Preferably, conditioning said first set of strands to a moisture content of between 25 and 40% based on the dry weight of said strands.
Preferably, said conditioning said second set of strands conditions said strands to a moisture content lower than 10% based on the dry weight of said strands.
Preferably, said conditioning said second set of strands conditions said strands to a moisture content of between 4 and 8% based on the dry weight of said strands.
Preferably, said pressing said mat lay-up between heated platens to form a consolidated panel is completed in at least 10% less time than that normally required for pressing based on the thickness of said panel.
Preferably, said pressing said mat lay-up between heated platens to form a consolidated panel is completed in at least 30% less time than that normally required for pressing based on the thickness of said panel.
Broadly the present invention also relates to a resin bonded consolidated panel made of wood strands said panel having an average density of between 450 and 550 kg/m3, a maximum density of at least 700 kg/m3 within 0.6 mm of the surface of said panel, an average MOE of at least 3000 MPa and an average MOR of at least 20 MPa and an internal bond of at least 0.28 MPa.